DES MOINES - The gap between the promises Iowa has made for public employees' retirement benefits and the money set aside to pay for them has grown to $5.7 billion - a 1,643 percent increase over 11 years, State Auditor David Vaudt said Monday.

"We had just a $327 million liability at the end of 2000. That has now grown to $5.7 billion, and that's equal almost to one year's general fund budget," Vaudt said. "We're going to need substantial resources in the future to improve the funded status of this particular plan."

A report last year by State Budget Solutions, a national nonprofit advocating for fundamental reform of state budgets, pegs Iowa's unfunded liability as even larger - $21.3 billion as of last March.

"The official estimate of the unfunded liability is based upon the state's promise to get a return on investment that is literally impossible," said State Budget Solutions editor Frank Keegan. "Taxpayers have to make up the difference, and that's going to cost a lot of money ... I'm worried that governors and legislatures don't really realize the magnitude of it."

Keegan said the average Iowa household will have to pay more than $800 in additional taxes every year for the next 30 years just for the existing unfunded pension promises. He said that's in addition to all other taxes and tax increases, no matter what pension reforms are enacted.

Assets in the Iowa Public Employees' Retirement System, or IPERS, were $23.2 billion as of June 30, 2011, up from $19.9 billion the previous year, IPERS spokeswoman Judy Akre said. The estimated value is a snapshot that fluctuates, depending on the stock market.